1. Field of the Invention
The present invention relates to a display-panel driving method for displaying an image in a desired manner.
2. Description of the Related Art
In recent years, as planar thin-type display panels of the matrix display system, plasma display panels (called ‘PDP’ hereinbelow) and electroluminescent display panels (called ‘ELDP’ hereinbelow) have been put to practical use. The light-emitting elements in the PDP and ELDP only have two states, namely a light-emitting state and a non-light-emitting state and, therefore, grayscale driving that employs the subfield method is implemented in order to obtain the intermediate grayscale faithful to an input picture signal.
In the subfield method, an input picture signal is converted to pixel data of N bits for each pixel and the display period of a single field is divided up into N subfields in correspondence with N bit digits of the N-bit pixel data. The numbers of light emissions for each bit digit of the pixel data are each allocated to each subfield. If one bit digit among the N bits is logic level 1, for example, light emission is executed the allocated number of times in the subfield corresponding to the bit digit concerned. On the other hand, when the bit digit is logic level 0, light emission is not implemented in the subfield corresponding with the bit digit. According to this driving method, the luminance of the intermediate grayscale corresponding with the total number of light emissions that are implemented in the respective subfields is visualized.
However, according to the grayscale driving based on this subfield method, the drop in contrast is striking when a particularly dark image is displayed. As a result, the whole of the screen becomes darker than desired.
In order to resolve this problem, a picture signal processing circuit that adjusts the signal level of the input picture signal in accordance with the peak level of the input picture signal has been proposed in Japanese Patent Kokai (Laid Open Application) No. 2000-13814. With this picture signal processing circuit, a brighter image is displayed by increasing the input picture signal level when the peak level of the input picture signal is low.
According to this picture signal processing, although the luminance can be increased, the number of luminance grayscales for low luminance portions in the image drops to the extent of this increase, and the image quality deteriorates.